Certainly I'm not saying that this is the algorithm to use in all cases. It's not even obvious how you would implement the sentinel algorithm in a language like

Certainly I'm not saying that this is the algorithm to use in all cases. It's not even obvious how you would implement the sentinel algorithm in a language like Python, not in idiomatic code anyway. The point is that the book is written from the perspective of an actual programmer writing actual code and thinking about how it will run. Granted, he's writing in a pixie assembly language for an imaginary machine, but still, the thought process is similar to writing real code. He doesn't just figure out the big-O and call it a day, he works out the low-level details. The lesson in this case is that a lot of extra work can be done in a loop, even for the simplest of algorithms. Now you might think, that's a truism, duh, any programmer knows that loops need to be fast. But what can I say? I read that section and spent a few hours thinking about it (even writing out some old-timey flowcharts), and I came out the other side writing better, faster code.